London-based Spider is the company that last year identified the Chameleon bot, which generated millions of fake ad impressions on hundreds of websites, defrauding advertisers. The tech blog CNET reported that Chameleon defrauded advertisers of about $6 million a month, in total.

All Things D reports that it’s significant that Google bought Spider “rather than build its own antibot technology.” Still, All Things D reports, Google already runs one of the least-infected advertising exchanges, AdEx.

In a prepared statement Google said advertiser confidence in the legitimacy of impression data is key to sustaining the web's core business model.

"To grow the pie for everyone, we need to take head on the issue of online fraud," Google said.